The subject matter disclosed herein relates to determining elevator car position. More particularly, the subject matter disclosed herein relates to determining elevator car position using bi-stable sensors.
It is known in the elevator art to define terminal zones at both ends of the elevator hoistway. The top landing of the building will normally be located within the top terminal zone as will the lower landing be located within the bottom terminal zone. It is desired that the elevator car stop normally at a top or bottom landing of the hoistway in such a terminal zone. As a safety measure, it is necessary to provide a number of backup means to ensure the elevator car does not collide with the mechanical hard-limits. Three levels of protection are usually provided when the elevator enters a terminal zone: the Normal Stopping Device, the Normal Terminal Stopping Device (or NTSD), and the Emergency Terminal Speed Limiting Device (or ETSLD). Embodiments of the invention may be used with NTSD which will take over from the Normal Stopping Device should the normal speed control signals fail to stop the car at the designated positions at the upper and lower ends of the hoistway. Two similar NTSDs are usually provided in the two terminal zones. One NTSD is installed at the bottom of the hoistway and one NTSD at the top of the hoistway. The NTSD system is designed to override the normal speed command signals and bring the car to stop at the terminal. It is also designed such that the NTSD terminal speed profile causes the slowdown pattern to be relatively smooth.
In order to implement the NTSDs, the position of the elevator car needs to be known by a control system. One existing method of determining elevator car position employs three sensors for detecting car position and a fourth sensor as a latching or clock input. The clock input indicates when the three sensors should be read to determine car position. As system noise can cause false clocking signals, improvements to such systems would be well received in the art. In addition, positions identified through the use of a simple binary code is sub-optimal in the required number of sense elements.